It's one of the main differentiators between instances. If you want no filters, you can make your own instance or see if you can find one with a "zero defederation" policy.

E.g. if you don't want to see a bunch of political propaganda or CSAM, and are into programming, programming.dev comes "pre configured" for that. Likewise lemmy.world, blahaj, etc... comes with their own flavour and configuration.

Yes, but users should be able to manually whitelist defederated instances so they can interact with such an instance, if they want to. Lemmygrad shouldn't be visible by default, but if you really like communist tankie shit, you should be able to manually allow it I guess.

Yes, because you don't want to provide them your content for free, so they can continue building engagement on your behalf.

Btw, was feddit.ch somehow defederated?

Yes, because end-user blocking only blocks posts from an instance, not its toxic users or their comments.